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            Influence of alloying elements 
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            Heat treatment

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Steel heat treatment is not without risk. The consequence of non-optimal control of the heat treatment process conditions is oxidization of the carbon, which is present in the periphery of the steel fastener. As a result, a thin layer with insufficient carbon may grow on the outside of the fastener. The mechanical properties of steel are determined to a high degree by the carbon it contains. As the affected layer has a lack of carbon, this layer cannot hardened and remains relatively soft. The pitch diameter of the screw thread is therefore severely weakened, which in turn substantially reduces the loadability of any threaded joints. Moreover, the thread friction experienced during assembly increases considerably, with the result being that the relationship between the tightening torque and the preload becomes unreliable.
This is a detrimental phenomenon for screw threads, as it unavoidably causes threads to strip when a load is applied. From the outside, it is impossible to see if decarburization has occurred. Additionally, threads do not always strip during assembly, so it could be noticed before disaster strikes.
Decarburization can only be detected with the microscopic or hardness tests described in EN ISO 898-1. In the picture from a schliff made for a hardness test to check for decarburization, the completely decarburized zone is clearly visible as a white area on the screw thread’s exterior.
To prevent this risk, fasteners are always quenched and tempered in a furnace that is supplied with a protective atmosphere (e.g., an inert gas) to ensure the carbon content remains at the required level for the type of steel being treated. Most furnaces used for quenching and tempering are also equipped with fire curtains in order to prevent oxygen from entering the furnace.
The standard EN ISO 898-1 contains very strict requirements concerning decarburization, which are summarized in the illustration below.
Although the standards allow for a certain degree of partial and complete decarburization, the limits are clearly set. Regardless of the fastener property class (8.8, 10.9 or 12.9), the depth (G) of the fully decarburized layer must not be more than 15 µm at the location of the pitch diameter. Furthermore, the standards specify the height E of the non-decarburized zone; e.g., for property class 8.8, this height must be at least ½ H, where H equals the height of the external thread in the maximal material condition.
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